Abstract
Abstract. The mean dynamic topography (MDT) is a key reference surface for altimetry. It is needed for the calculation of the ocean absolute dynamic topography, and under the geostrophic approximation, the estimation of surface currents. CNES-CLS mean dynamic topography (MDT) solutions are calculated by merging information from altimeter data, GRACE, and GOCE gravity field and oceanographic in situ measurements (drifting buoy velocities, hydrological profiles). The objective of this paper is to present the newly updated CNES-CLS18 MDT. The main improvement compared to the previous CNES-CLS13 solution is the use of updated input datasets: the GOCO05S geoid model is used based on the complete GOCE mission (November 2009–October 2013) and 10.5 years of GRACE data, together with all drifting buoy velocities (SVP-type and Argo floats) and hydrological profiles (CORA database) available from 1993 to 2017 (instead of 1993–2012). The new solution also benefits from improved data processing (in particular a new wind-driven current model has been developed to extract the geostrophic component from the buoy velocities) and methodology (in particular the computation of the medium-scale GOCE-based MDT first guess has been revised). An evaluation of the new solution compared to the previous version and to other existing MDT solutions show significant improvements in both strong currents and coastal areas.
Highlights
The estimation of an accurate mean dynamic topography (MDT) has been a long-standing issue for the reconstruction of the absolute dynamic topography from altimeter data (Rio, 2010)
The lack of an accurate geoid at spatial scales corresponding to the along-track spatial resolution of altimeter data (7 km at 1 Hz, 300 m at 20 Hz) has led to the exploitation of the time-variable part of the sea level with respect to the sea level mean over a given reference period: the sea level anomaly (SLA)
We propose an adjustment of the mean geostrophic surface velocities from Koenig et al (2014) (LUT – updated, red dashed line) close to the South American continent to account for this coastal current associated with a northern branch of the Subantarctic Front (SAF)
Summary
The estimation of an accurate mean dynamic topography (MDT) has been a long-standing issue for the reconstruction of the absolute dynamic topography from altimeter data (Rio, 2010). For years, the use of altimetry for scientific ocean studies has focussed on the analysis of sea level anomalies. While providing invaluable insight into the ocean dynamics of mesoscale eddies, a large number of scientific and operational activities rely on the accurate estimate of the absolute sea level. The absolute dynamic topography is directly linked, under the geostrophic assumption, to the ocean surface currents.
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